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<h1><a name="SECTION0017100000000000000000">
15.1 <tt class="module">select</tt> --
         Waiting for I/O completion</a>
</h1>

<p>
<a name="module-select"></a>

<p>
This module provides access to the <tt class="cfunction">select()</tt>
and <tt class="cfunction">poll()</tt> functions
available in most operating systems.  Note that on Windows, it only
works for sockets; on other operating systems, it also works for other
file types (in particular, on <span class="Unix">Unix</span>, it works on pipes).  It cannot
be used on regular files to determine whether a file has grown since
it was last read.

<p>
The module defines the following:

<p>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-3392' xml:id='l2h-3392' class="exception">error</tt></b></dt>
<dd>
The exception raised when an error occurs.  The accompanying value is
a pair containing the numeric error code from <tt class="cdata">errno</tt> and the
corresponding string, as would be printed by the C function
<tt class="cfunction">perror()</tt>.
</dd></dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3393' xml:id='l2h-3393' class="function">poll</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
(Not supported by all operating systems.)  Returns a polling object,
which supports registering and unregistering file descriptors, and
then polling them for I/O events;
see section&nbsp;<a href="poll-objects.html#poll-objects">15.1.1</a> below for the methods supported by
polling objects.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-3394' xml:id='l2h-3394' class="function">select</tt></b>(</nobr></td>
  <td><var>iwtd, owtd, ewtd</var><big>[</big><var>, timeout</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This is a straightforward interface to the <span class="Unix">Unix</span> <tt class="cfunction">select()</tt>
system call.  The first three arguments are sequences of `waitable
objects': either integers representing file descriptors or
objects with a parameterless method named <tt class="method">fileno()</tt> returning
such an integer.  The three sequences of waitable objects are for input,
output and `exceptional conditions', respectively.  Empty sequences are
allowed, but acceptance of three empty sequences is platform-dependent.
(It is known to work on <span class="Unix">Unix</span> but not on Windows.)  The optional
<var>timeout</var> argument specifies a time-out as a floating point number
in seconds.  When the <var>timeout</var> argument is omitted the function
blocks until at least one file descriptor is ready.  A time-out value
of zero specifies a poll and never blocks.

<p>
The return value is a triple of lists of objects that are ready:
subsets of the first three arguments.  When the time-out is reached
without a file descriptor becoming ready, three empty lists are
returned.

<p>
Among the acceptable object types in the sequences are Python file
objects (e.g. <code>sys.stdin</code>, or objects returned by
<tt class="function">open()</tt> or <tt class="function">os.popen()</tt>), socket objects
returned by <tt class="function">socket.socket()</tt>.<a id='l2h-3396' xml:id='l2h-3396'></a><a id='l2h-3398' xml:id='l2h-3398'></a>You may also define a <i class="dfn">wrapper</i> class yourself, as long as it has
an appropriate <tt class="method">fileno()</tt> method (that really returns a file
descriptor, not just a random integer).
<span class="note"><b class="label">Note:</b>
File objects on Windows are not acceptable, but sockets
are.<a id='l2h-3399' xml:id='l2h-3399'></a>  On Windows, the underlying <tt class="cfunction">select()</tt>
function is provided by the WinSock library, and does not handle file
descriptors that don't originate from WinSock.</span>
</dl>

<p>

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